Microwave circuits for accumulating power output from multiple negative resistance diode oscillators are well known in the art, for example, the patents of Kurokawa et al, U.S. Pat. No. 3,628,171 and Harp et al, "Power Combining of X-Band IMPATT Circuit Modules," 1973, IEEE International Solid-State Circuits Conference, pp 118-119, show such devices. The general device type utilizes a negative resistance diode oscillator in one end of a coaxial microwave circuit. The coaxial circuit includes a transformer for matching the diode to the characteristic impedance of the coaxial line and a matched load resistance at the end opposite the diode for stabilization of the diode. The coaxial line may be coupled to a resonant cavity at a point intermediate to the diode and the matched load. This cavity may be used to accumulate power from several diodes, each mounted in its own coaxial circuit. The RF energy may then be taken by means of a probe inserted into the resonant cavity.
These circuits typically suffer from several disadvantages; for example, the output efficiency depends upon the ratio of the real part of the input impedance looking into the resonant cavity and the value of the terminating matched resistance. This efficiency improves with an increasing ratio. The maximum real input impedance of the resonant cavity occurs at the resonant frequency and decreases rapidly, with a change in frequency since the cavity is of high Q construction. This means that in order to obtain maximum efficiency the diode has to be made resonant by the coaxial circuit external to the resonant cavity. In theory this can be done, but, in practice it becomes very difficult. Further, if it is necessary to combine several diodes, each diode has to undergo a tedious optimization process since no two microwave semiconductor diodes are alike.
Further, these circuits make no provision for controlling second harmonic loading. Such control is essential for more efficient output power and for reduced noise level operation. Still further, the circuits will suffer in efficiency when attempts are made to tune them.